Skip sniffing and reseting if retry is disabled

Bypass sniffing entirely if retries has been disabled.

Fix bug #1216981

Change-Id: I593bdc56ca139af5a7f2ca2783ef2de2a96c94fb
Signed-off-by: Yuan Zhou <yuan.zhou@intel.com>
Don't crash when header is value of None

Before commit 7d88d14d (http://git.io/hhJWdQ) swifclient used to accept
header value with the type None :

{'Headers': None}

It would just be happy with it with those None headers and not process
them, reinstate the old behavior.

Closes-Bug: 1256441
Change-Id: Ic7f80c38ef2ce9ef1687ed0d6d65521f2e754905
Merge "Don't crash when header is value of None"
Merge "Skip sniffing and reseting if retry is disabled"